This invention relates to a method for working the surface of grain-oriented silicon steel to affect the domain size and reduce core losses. More particularly, this invention relates to providing localized strains on the surface of grain-oriented silicon steel by using pressurized liquid jets.
In the manufacture of grain-oriented silicon steel, it is known that secondary recrystallization texture, e.g., Goss texture (110)[001], in accordance with Miller's indices, results in improved magnetic properties, particularly permeability and core loss. The Goss texture refers to the body-centered cubes making up the grain or crystals being oriented in the cube-on-edge position. The texture or grain orientations of this type refers to the cube edges being parallel to the rolling direction and in the plane of rolling, and the cube face diagonals being perpendicular to the rolling direction and in the rolling plane. As is well known, steels having this orientation are characterized by a relatively high permeability in the rolling direction and a relatively low permeability in a direction at right angles thereto.
In the manufacture of grain-oriented silicon steel, typical steps include providing a melt on the order of 2-4.5% silicon, casting the melt, such as by a continuous casting process, hot rolling the steel, cold rolling the steel to final gauge with an intermediate annealing when two or more cold rollings are used, decarburizing the steel, applying a refractory oxide base coating, such as magnesium oxide coating, to the steel, and final texture annealing the steel at elevated temperatures in order to produce the desired secondary recrystallization and purification treatment to remove impurities, such as nitrogen and sulfur. The development of the cube-on-edge orientation is dependent upon the mechanism of secondary recrystallization wherein during recrystallization, secondary cube-on-edge oriented grains are preferentially grown at the expense of primary grains having a different and undesirable orientation.
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, distribution transformers, generators and the like. The silicon content of the steel in electrical applications permits cyclic variation of the applied magnetic field with limited energy loss, which is termed core loss. It is desirable, therefore, in steels of this type to reduce core loss.
It is known that core loss values of grain-oriented silicon steels may be reduced if the steel is subjected to any of various practices to induce localized strains in the surface of the steel. Such practices may be generally referred to as "scribing" and may be performed either prior to or after the final high temperature annealing operation. If the steel is scribed after the decarburization anneal but prior to final high temperature texture anneal, then the scribing generally controls the growth of the secondary recrystallization grains to preclude formation of large grains and so results in reduced domain sizes. U.S. Pat. No. 3,990,923, issued Nov. 9, 1976, discloses methods wherein prior to the final high temperature annealing, a part of the surface is worked, such as by mechanical plastic working, local thermal treatment, or chemical treatment.
If the steel is scribed after final texture annealing, then there is induced a superficial disturbance of the stress state of the texture annealed sheet so that the domain wall spacing is reduced. These disturbances typically are relatively narrow, straight lines, or scribes generally spaced at intervals equal to or less than the grain size of the steel. These scribe lines are typically transverse to the rolling direction and typically applied to only one side of the steel. U.S. Pat. No. 3,647,575, issued Mar. 7, 1972, discloses a method wherein watt losses are to be improved in cube-texture silicon-iron sheets after annealing and complete recrystallization. The method includes partially plasticly deforming the sheet surface by providing narrowly spaced shallow grooves, such as by a cutter or abrasive powder with pressure applied. The sheet is preferably scribed on opposite sides in different directions.
There have also been attempts to improve the magnetic properties of steel after final texture annealing by projecting particles, such as steel shots, onto substantially linear selected portions of a grain-oriented steel sheet to produce strains in the regions. U.S. Pat. No. 4,513,597, issued Apr. 30, 1985, discloses an apparatus including an endless belt loop in which slits are formed at a predetermined distance and elongated in the direction perpendicular to the path of travel and movable at the speed synchronously with the speed of the steel sheet. The apparatus includes a means for projecting particles through the slits and against the steel sheet.
In the use of such grain-oriented silicon steels during fabrication incident to the production of transformers, for example, the steel is cut and subjected to various bending and shaping operations which produce stresses in the steel. In such instances, it is necessary and conventional by manufacturers to stress relief anneal the product to relieve such stresses. During stress relief annealing, it has been found that the beneficial effect on core loss resulting from some scribing techniques, such as thermal scribing, are lost.
What is needed is a method for reducing the core loss values over that which are available to grain-oriented silicon steels which are not subjected to scribing, i.e., which are only final texture annealed. It is desirable that a method be developed for scribing such steel wherein the scribe lines required to improve the core loss values of the steel may be applied in a uniform and efficient manner to result in uniform and reproducably lower core loss values. A low cost scribing practice should be compatible with the conventional steps and equipment for producing grain-oriented silicon steels, and, furthermore, such improvements in core loss values should be able to survive stress relief annealing which are incident to the fabrication of such steels into end product.